The present invention relates to a method of fabricating an electronic device suitable for single-electron devices, various quantum effect devices and the like as well as to the electronic device fabricated by such a method.
In order to improve the performance of the electronic device, such as a silicon integrated circuit (LSI), fine-structuring of elements constituting the circuit is being advanced. However, MOS transistors mainly used at present in the devices have a limitation in improving integration and operation speed by this fine-structuring while suppressing increased power consumption.
Recently, in order to solve the limitation, single-electron elements have been proposed as electronic elements based on a new operation principle. When the elements are realized ideally, it is expected that the power delay product can be improved greatly. The single-electron element is described in, for example, APPLIED PHYSICS, Vol. 63, No. 12 (1994), pp. 1232-1238. Further, it is considered that the 0-dimensional quantum box for confining electrons in an area of several nm three-dimensionally can be used to improve the performance of light emitting elements or the like greatly by the quantum effect. In addition, it is considered that a one-dimensional quantum line can be used to form one-dimensional electron gas, so that the mobility of electrons can be increased greatly and a high-speed switching element can be realized. The 0-dimensional quantum box ideally means that electrons are fixed at one point of coordinates and the one-dimensional quantum line ideally means that electrons can be moved only in one direction. The quantum effect elements are discussed in, for example, the report of the Institute of Electronics and Communication Engineers of Japan, Vol. 77, No. 11 (1994), pp. 1117-1124.
However, as discussed in the above paper, in order to operate the single-electron device at room temperature, it is necessary to fabricate a device structure having a size of several nm which is smaller by one or two orders than several hundred nm for the size of the currently leading MOS transistor with accuracy. The quantum effect element such as the 0-dimensional quantum box and the one-dimensional quantum line are also the same.
For example, it is considered that an electron storage node is connected through two tunnel junctions to an external wiring. When a voltage is applied from the external wiring, electrons attempt to pass through the electron storage node through the tunnel junctions. However, energy of the electron storage node is increased by the storage energy for one electron in a short time that the electron passes through the electron storage node and consequently a next electron is prevented from entering the storage node subsequently. Thus, for example, by disposing a gate electrode to change a potential of the electron storage node by application of a voltage from the gate, a tunnel current passing through the storage node can be controlled. However, in order to attain such control at the room temperature, the energy in case where one electron is stored in the storage node must be sufficiently larger than thermal noise.
In other words, e.sup.2 /2C&gt;kT and accordingly the following equation must be satisfied. EQU C(aF)&lt;929/T(K)
where e represents an elementary charge, C represents a capacitance of the electron storage node, T represents an operating temperature in Kelvin, k represents the Boltzmann's constant, and a of aF represents an abbreviation of "atto-" meaning 10.sup.-18. When a sectional area, a thickness and a dielectric constant of the two tunnel junctions are S, d and e, respectively, the following equation is given using C=2.epsilon.S/d EQU S&lt;e.sup.2 d/4.epsilon.kT
For example, when the thickness of tunnel insulative layer is 2 nm, the sectional area S of the tunnel junctions is required to be made sufficiently smaller than 200 nm.sup.2 in order to satisfy the above condition at the room temperature. That is, it is necessary to form tunnel junctions having a sectional area of at least 100 nm.sup.2 or less, preferably several nm.sup.2 to several tens nm.sup.2.
At present, there is no fabricating apparatus capable of attaining such super-fine structures and furthermore there is scarcely any prospect capable of attaining mass production with good reproductivity. Further, in order to achieve stable operations for the single electron devices, it is desirable to form a multi-tunnel junction having tunnel junctions formed in series, while a more complicated fabricating process is required therefor and it is difficult to cope with the process by the current lithography technique.
Furthermore, an experimental result that thin layers are laminated on a substrate to form tunnel junctions is reported in Journal of Non-Crystalline Solids, 128 (1991), 91-100. However, there is no teaching as to how transistors, memories and light emitting elements are fabricated when the laminated structure is formed on the substrate in the vertical direction to the substrate to form the tunnel junctions. A relevant memory technique is disclosed in U.S. Ser. No. 291,752 filed Aug. 16, 1994 and assigned to the same assignee as the present invention (U.S. Pat. No. 5,600,163).